The present invention generally related to error handling in the field of communication systems and, more particularly to error handling using forward error correction (FEC), automatic retransmission requests (ARQ) and variable redundancy in digital communication systems.
Reference is made to a related application, xe2x80x9cMethod and System for Measurement Based Automatic Retransmission Request in a Radiocommunication Systemxe2x80x9d which was filed as U.S. patent application Ser. No. 09/168,064 on Oct. 8, 1998, the contents of which are hereby incorporated by reference in its entirety.
The growth of commercial communication systems and, in particular, the explosive growth of cellular radiotelephone systems, have compelled system designers to search for ways to increase system capacity without reducing communication quality beyond consumer tolerance thresholds. One technique to achieve these objectives involved changing from analog modulation based systems to digital modulation based systems.
In wireless digital communication systems, industry standards define most of the system parameters, including, for example, modulation type, burst format, communication and protocol. For example, the European Telecommunication Standard Institute (ETSI) established the Global System for Mobile Communications (GSM) standard that uses time division multiple access (TDMA) to transmit control, voice and data information over radio frequency (RF) physical channels or links and a Gaussian Minimum Shift Keying (GMSK) modulation scheme at a symbol rate of 271 kilosymbols per second (ksps). In the United States, the Telecommunication Industry Association (TIA) has published a number of Interim Standards, such as IS-54 and IS-136, that define various versions of Digital Advanced Mobile Phone Service (D-AMPS), a TDMA system that uses a Differential Quadrature Phase Shift Keying (DQPSK) modulation scheme for communicating data over RF links.
TDMA systems subdivide the allocated frequency into one or more RF channels. Each of the RF channels is further divided into a number of time frames. Each time frame is then divided into a number of timeslots, (e.g., three timeslots), wherein each timeslot corresponds to a physical channel. Logical channels are then formed from one or several physical channels. In these systems the mobile stations communicate with one or more base stations by transmitting and receiving bursts of digital information over uplink and downlink RF channels.
Digital communication systems employ various techniques to handle erroneously received information. One such technique is FEC. In general, FEC involves transmitting additional bits that are used at the receiving end to verify the accuracy of the transmission, and if necessary, correct any errors. FEC techniques involve convolutional or block coding of the data prior to modulation, wherein it is common to refer to convolutional codes by a code rates (e.g., xc2xd and ⅓), wherein a lower code rates involves a greater number of code bits. Therefore, a lower code rate typically provides greater error protection. However, it also results in a lower user bit rate.
The technique used to select a code rate is called Link Adaptation (LA). LA works in conjunction with FEC by monitoring the quality of the channel or link and adjusting the code rate accordingly. For example, if the quality of the link is low, the code rate will be lowered. Alternatively, if the quality of the link is high, the code rate may be raised in order to provide a higher user bit rate.
Another common technique for handling erroneously received information is known as ARQ. In general, ARQ involves analyzing a received block of data for errors at the receiver and requesting that the sender retransmit the block of data if errors are detected. When processed by a receiver (e.g., a receiver in a mobile radio telephone), each block can, after demodulation, be evaluated for errors using a block check sequence and well known cyclic redundancy check techniques. If there are errors, then a request is sent back to the transmitting entity (e.g., a base station in a radiocommunication system) denoting the block to be resent using predefined ARQ protocols.
As one skilled in the art will appreciate, FEC techniques (e.g., FEC techniques including LA) may be combined with ARQ techniques. Such combined techniques are commonly referred to as hybrid ARQ techniques. Hybrid ARQ techniques permit correction of some received errors using FEC coding at the receiver, while correction of other errors may require retransmission.
FIG. 1 illustrates an exemplary hybrid ARQ scheme, known as Type I Hybrid ARQ. Type I Hybrid ARQ is used in conjunction with General Packet Radio Service (GPRS), wherein four FEC Modulation and Coding Schemes (MCS), CS-1 through CS4 are employed with coding rates of xc2xd, ⅔, xc2xe, and 1, respectively. After one of the four FEC MCS is selected (using LA) for a current Logical Link Control (LLC) frame 110, segmentation of this frame to Logical Link Control (LLC) blocks, corresponding to the selected coding scheme, is performed. The LLC blocks include a payload of information 111, a frame header (H1) and a frame check sequence (FCS). The LLC blocks are coded with the selected rate forming a coded block 112. In order to reduce the number of bits in the coded block, bits can be removed using a known puncturing pattern to form the punctured coded block that will be used as the radio link control (RLC) block 113. If an RLC block is found to be erroneous at the receiver (i.e., it has errors which cannot be corrected) and needs to be retransmitted, the originally selected FEC coding (and puncturing) scheme is used for retransmission (i.e., this system employs fixed redundancy for retransmission purposes).
Another exemplary hybrid ARQ scheme, known as Incremental Redundancy (IR) or Type II hybrid ARQ, provides for additional redundant bits to be transmitted if the originally transmitted block cannot be decoded. This scheme is conceptually illustrated in FIG. 2. Therein, multiple decoding attempts are made by the receiver. First the receiver attempts to decode the originally received data block. If the receiver is unable to decode the originally received data block, the receiver sends a retransmission request to the sender. The receiver then receives additional redundant block R1, which it uses in conjunction with the originally transmitted data block to attempt decoding. The probability of decoding is increased due to the diversity of the two transmitted blocks. The Type II Hybrid ARQ retransmission (R1, R2) is optimized to be decoded in combination with previous transmissions and may or may not be separately decodable. If the receiver still cannot decode the data block, the receiver obtains another block of redundant information R2, which it uses in conjunction with the originally received data block and the block of redundant bits RI to attempt decoding for a third time. This process can be repeated until successful decoding is achieved.
For a further discussion of Type II Hybrid ARQ, reference is made to xe2x80x9cComplementary Punctured Convolutional (CPC) Codes and Their Applications,xe2x80x9d by S. Kallel in IEEE Transactions on Communications, volume 23, number 6, published June 1995, the content of which is hereby incorporated by reference in its entirety.
Presently, using the above techniques, if a receiver using Type I Hybrid ARQ attempts to communicate with a transmitter using Type II Hybrid ARQ, assuming the coding scheme of the first transmitted block was known to the receiver, the receiver would not be able to decode any retransmission. In addition, even if the coding scheme for the retransmission was communicated to the Type I Hybrid ARQ receiver, the quality of service would be greatly diminished since the Type I Hybrid ARQ receiver will discard previous transmissions and the retransmissions are not optimized to be separately decodable.
Therefore, it is desirable for a transmitter to operate using a scheme that will allow receivers using either Type I Hybrid ARQ or receivers using Type II Hybrid ARQ to successfully receive and decode blocks from a common transmitter without the need for the transmitter to know which scheme is being employed by the receivers.
The present invention describes a transmitter that operates using a scheme that allows receivers using either Type I Hybrid ARQ or receivers using Type II Hybrid ARQ to successfully receive and decode blocks from a common transmitter without the need for the transmitter be informed of which scheme is being employed by the receivers.
According to a first aspect of the present invention, provided is a system for communicating blocks of information to and from a transmitter and a receiver. The system comprises a transmitter that operates in an ARQ transmission mode and a plurality of receivers including a first receiver that operates in a first reception mode and a second receiver that operates in a second reception mode. The first receiver and the second receiver communicate with the transmitter while the transmitter operates in the ARQ transmission mode.
According to a further aspect of the present invention, provided is a communication device that operates in an ARQ mode. The communication device comprises a memory that stores a plurality of coding schemes in a predetermined order. The communication device also includes a receiver that receives a block and one or more retransmitted blocks. The receiver individually processes each of the one or more retransmitted blocks using a different coding scheme corresponding to the predetermined order of the coding schemes stored in the memory. In addition, the block and each of the one or more retransmitted blocks are coded using the plurality of coding schemes and have a substantially equal probability of being independently decoded by the communication device.
According to a further aspect of the present invention, provided is a method of transmitting and receiving blocks in a communication system. The method comprises transmitting the blocks in an ARQ transmission mode; receiving the blocks, at a first receiver, operating in a first reception mode; and receiving the blocks, at a second receiver, operating in a second reception mode.
According to a further aspect of the present invention, provided is a method of receiving a block from a transmitter. The method comprises storing a plurality of coding schemes in a predetermined order; receiving, at a receiver, the block and one or more retransmitted blocks from the transmitter; and individually processing, at the receiver, each of the one or more retransmitted blocks using a different coding scheme corresponding to the predetermined order of the plurality of coding schemes. Each block and each of the one or more retransmitted blocks are coded using the plurality of coding schemes and have a substantially equal probability of being independently decoded by the receiver.